US military guards move a detainee to an undefined facility inside Camp Delta in the Guantanamo Bay prison. (AFP file photo)

US President Barack Obama says he remains hopeful of being able to fulfill his eight-year-old promise of closing the US military prison at Guantanamo Bay, Cuba, before his term ends in January.

Obama was asked about the future of Guantanamo on Thursday as he conducted a news conference in the capital of Laos while wrapping up his final visit as president to Asia.

“I am not ready to concede that it may still remain open because we’re still working diligently to continue to shrink the population,” Obama said following a summit with leaders of the Association of Southeast Asian Nations (ASEAN) in Vientiane.

“It’s not necessary and it’s hugely expensive for taxpayers,” the president added.

“As we continue to shrink the population to the point where we’re looking at 40 or 50 people and are maintaining a multi-million dollar operation to house these people, the American people are asking why should we spend this money on this when it could be spent on other things.”

The Guantanamo prison and its associated military commissions cost the Pentagon $445 million in fiscal year 2015. That means each of the 61 prisoners left at the facility cost more than $6 million per year.

About 780 men have passed through the facility since it was opened following the September 11, 2001 terror attacks to hold terrorism suspects.

In recent months, the Obama administration has accelerated the rate at which detainees who have been approved for transfer are released from the detention center.

Obama had promised to close the Guantanamo prison during the 2008 presidential election campaign, but his efforts have been continually thwarted by Republican lawmakers in Congress.

The US presidential election in November will likely determine the future of Guantanamo, as Republican presidential nominee Donald Trump has promised to fill the notorious prison with “bad dudes” if he is elected president.